Install the project using:
pip install django-polymorphic
Update the settings file:
INSTALLED_APPS += ( 'polymorphic', 'django.contrib.contenttypes', )
The current release of django-polymorphic supports Django 1.8, 1.10, 1.11 and Python 2.7 and 3.4+ is supported. For older Django versions, use django-polymorphic==1.2.
Making Your Models Polymorphic¶
PolymorphicModel instead of Django’s
models.Model, like so:
from polymorphic.models import PolymorphicModel class Project(PolymorphicModel): topic = models.CharField(max_length=30) class ArtProject(Project): artist = models.CharField(max_length=30) class ResearchProject(Project): supervisor = models.CharField(max_length=30)
All models inheriting from your polymorphic models will be polymorphic as well.
Using Polymorphic Models¶
Create some objects:
>>> Project.objects.create(topic="Department Party") >>> ArtProject.objects.create(topic="Painting with Tim", artist="T. Turner") >>> ResearchProject.objects.create(topic="Swallow Aerodynamics", supervisor="Dr. Winter")
Get polymorphic query results:
>>> Project.objects.all() [ <Project: id 1, topic "Department Party">, <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">, <ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
not_instance_of for narrowing the result to specific subtypes:
>>> Project.objects.instance_of(ArtProject) [ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner"> ]
>>> Project.objects.instance_of(ArtProject) | Project.objects.instance_of(ResearchProject) [ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">, <ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
Polymorphic filtering: Get all projects where Mr. Turner is involved as an artist or supervisor (note the three underscores):
>>> Project.objects.filter(Q(ArtProject___artist='T. Turner') | Q(ResearchProject___supervisor='T. Turner')) [ <ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">, <ResearchProject: id 4, topic "Color Use in Late Cubism", supervisor "T. Turner"> ]
This is basically all you need to know, as django-polymorphic mostly works fully automatic and just delivers the expected results.
Note: When using the
dumpdata management command on polymorphic tables
(or any table that has a reference to
--natural flag in the arguments. This makes sure the
ContentType models will be referenced by name
instead of their primary key as that changes between Django instances.
While django-polymorphic makes subclassed models easy to use in Django,
we still encourage to use them with caution. Each subclassed model will require
Django to perform an
INNER JOIN to fetch the model fields from the database.
While taking this in mind, there are valid reasons for using subclassed models.
That’s what this library is designed for!